Process and composition for treating



United States Patent PROCESS AND COMPOSITION FOR TREATING MOLTENALUMINUM Hugh S. Cooper, Shaker Heights, ()hio, assignor to Walter M.Weil, Cleveland, Ghio No Drawing. Application May 29, 1953, Serial No.358,577

3 Claims. (CI. 75-68) This invention relates to the production ofaluminum and aluminum alloys containing small amounts of bothtitaniumand boron as grain refining constituents and for other purposes. Theinvention involves both a novel method of introducing the titanium andboron into an aluminum melt and a novel combination of titanium andboron in gaseous form in a pressure tank or cylinder from which thegases may be discharged directly into molten aluminum.

Titanium has long been added to aluminum and its alloys as a grainrefining alloying agent. More recently, the addition of boron toaluminum and its alloys for similar purposes has been proposed and hasbeen found to possess certain advantages over titanium. One of theadvantages obtained by the use of boron is that the resulting aluminumalloys retain the grain refining effects to a greater degree afterseveral remeltings.

Titanium has been added to aluminum and its alloys in commercialoperations by two principal methods. One of the methods was to prepare amaster alloy of aluminum and titanium containing a relatively highpercentage of titanium and to add the master alloy to an aluminum melt.The other method was to introduce the titanium into the aluminum melt inthe form of gaseous titanium tetrachloride. The master alloy method isexpensive to employ in commercial production because of the diflicultyof preparing and the high cost of the master alloy. The titaniumtetrachloride method has the great advantage that it effectively degasesand cleans the melt so treated. However, it has not been a convenientmethod to employ in commercial operations because titanium tetrachlorideboils at about 136.4 C. at normal pressure, and it is normally necessaryto maintain this reagent substantially above that temperature tointroduce it into a melt in gaseous form. Since safety regulationsrequire the use of safety plugs in gas cylinders which fuse at about 72C., the required gas temperature in a titanium tetrachloride tank orcylinder far exceeds the upper limit imposed by safety regulations andprevents commercial use of this reagent from gas cylinders.

While boron trichloride boils at about 13 C. and is thus easily suppliedas a gas from pressure tanks or cylinders at temperatures providing alarge margin of safety, this reagent is many times more expensive thantitanium tetrachloride. Its use in treating aluminum melts has probablybeen limited by its cost more than by any other consideration, and ithas been little used in commercial operations, to the best of myknowledge. The solubility of boron in aluminum is so low that the masteralloy method of introducing it into an aluminum melt is not feasible,and other methods of introducing the boron, such as by the inclusion ofboron salts in a melting flux, have been difficult to control because ofthe tendency of the boron salts to remain in the flux rather than to bereduced and alloyed with the aluminum.

Some information has been published on the value of incorporating bothtitanium and boron in aluminum and "ice its alloys. Use of both of thesealloying elements has been shown to produce a finer grain structure thanwhen either element alone was used. However, the problem of introducingthe two alloying elements in a satisfactory manner for commercialoperations has involved many difiiculties. Insofar as I am aware,introduction of the two alloy metals has only been successfullyperformed by adding boron to an aluminum alloy in which the desiredamount of titanium has already been incorporated. This procedure hasinvolved the same objections as the incorporation of boron alone intoaluminum, and is additionally objectionable because it involves a secondoperation.

In view of the above, I have sought to develop a satisfactory method forintroducing titanium and boron together directly into an aluminum meltin the form of their gaseous chlorides (TiClr and BCls). By substitutinga substantial amount of titanium tetrachloride for the expensive borontrichloride, the cost of the latter becomes less significant.

Since titanium tetrachloride and boron trichloride are respectivelynormally liquid and normally gaseous and have widely different boilingpoints (about 136 C. and 13 C., respectively), the problem ofintroducing them into an aluminum melt together in fixed proportions hadno obvious solution. This problem also involved the diificulty ofsupplying the two gases at a pressure high enough to force them intolarge aluminum melts against the pressure existing adjacent the bottomsof the melts without requiring that the supply tanks be held at atemperature too close to the limit imposed by the melting point of thsafety plugs required by safety regulations.

In the course of working on this problem, I made the surprisingdiscovery that a mixture of liquid titanium tetrachloride and liquidboron trichloride, in fixed proportions, can be discharged from the samegas cylinder as mixed gases at temperatures considerably below theboiling point of titanium tetrachloride alone. However, I also foundthat the proportions in which the two gases are discharged are notnecessarily the same as the propor tions in which the compounds arepresent in the cylinder and that the proportions discharged and thecylinder pressures vary with the temperature at which they are releasedand also with time as the proportions of the compounds left in thecylinder change during the process.

Finally, I discovered that mixtures of the two compounds, whenmaintained at the proper temperature, will release a mixture of the twocompounds in' gaseous form in substantially the same proportions inwhich they are contained in a gas cylinder and that these proportionswill remain substantially constant if this temperature is maintained,until the cylinder is substantially exhausted. For example, when amixture of titanium tetrachloride and boron trichloride in about equalproportions by weight is held in liquid form in a cylinder and thecylinder is maintained at a temperature of about 60 C., the twocompounds can be released from the cylinder in gaseous form insubstantially the same proportions until the cylinder is substantiallyexhausted. Under these conditions, the liquid mixture maintains a gaspressure in the cylinder of about 39 pounds per square inch, which ismore than ample for forcing the mixed gases through large aluminum meltsin most commercial operations.

remains principally in liquid form until the tank approaches exhaustion.

The preferred method of treating melts of aluminum and its alloys inaccordance with the present invention is to employ a conventional gascylinder as a pressure tank and to charge the cylinder with measuredquantities of the two chlorides while holding them below the boilingpoint of the boron trichloride, i. e. below 13 C. Such cylinders, areconventionally equipped with outlet conduits having control valvestherein, and a gas line may be connected to the outlet conduit and rundownwardly into a molten metal bath in a furnace to a region adjacentthe bottom thereof. The gas cylinder is preferably maintained at anoperating temperature of 50 to 60 C. by immersing the tank in a waterbath, oil bath, or the like, equipped with suitable thermostaticallycontrolled heating elements, whereby the liquid contents of the cylindermay be maintained at a substantially constant temperature in spite ofthe cooling effect of evaporation as the mixed gases, are discharged.

When a molten metal bath to be treated has been prepared in the furnace,the gas line from the gas cylinder may be inserted therein, and thevalve of the cylinder may be cracked to bleed the gas mixture into themelt.

The mixed gases discharged into the melt bubble upwardly therethroughand act to degasify and clean up the melt by removing hydrogen and otherimpurities. Some of the titanium and boron remain in the melt and exerta profound grain refining effect. Since there is a distinct limit on theamount of titanium and boron that can be retained in the melt by thisprocedure, there is no limit on the length of time the gassing operationcan be continued. The amount of gassing required in any particular casewill be determined by the nature and purity of the product sought. Also,difierent alloys require ditlerent amounts of gas treatment to becleaned up and refined by this procedure; and the initial purity of themetal from which the melt is made greatly afiects the extent of gastreatment required to produce a satisfactory product.

The invention has been employed according to the above-described mode ofoperation with outstanding success in the production of aluminum alloycastings. The castings resulting from use of the invention in thismanner have shown a very line and uniform grain size and have beenunusually free from the porosity commonly caused by gas inclusions.

Depending upon the gas pressures required, which are in turn determinedby the depth of a molten metal bath to be treated, the proportions ofthe two reagents in the pressure cylinder may be varied to some degree.The temperature required to evaporate the mixed gases in substantiallythe same proportions as the reagents are present in the cylinder willalso vary with the proportions employed. Within the practically usefulrange of pressures, however, and while operating well below the safetyplug temperature limit of about 72 C., the proportions of the tworeagents must not depart too far from equal proportions, though somevariation is permissible. For example, a mixture of about 44% titaniumtetrachloride and 56% boron trichloride by weight worked satisfactorilyat about 25 pounds per square inch.

For obvious practical reasons, it isdesirable for most purposes that thetwo gases be discharged from the cylinder in substantially the sameproportions as the liquid reagents are contained in the cylinder.However, to the best of my knowledge, it has never before beenrecognized that the two reagents could be discharged together from thesame pressure compartment in any proportions. Because it may be desiredin some instances to discharge the two gases in different proportionsfrom a given cylinder, and because of the difliculty of accuratelyproportioning the two chemicals in a pressure cylinder in the firstinstance, considerable departure from the approximately equalproportions of the two gases, preferred for most purposes, may beaccidentally or intentionally encountered in practice. Such departures,nevertheless, still involve use of the basic discovery on which thisapplication is based, i. e. that the two reagents may be discharged asmixed gases from a liquid mixture held under pressure well below theboiling temperature of the titanium tetrachloride. By proper regulationof the temperature both gases will be discharged simultaneously untilthe cylinder is substantially exhausted.

The invention involves evaporating a liquid mixture of the boron andtitanium chlorides at a temperature which is substantially below theboiling point of one of the compounds alone. Insofar as I am aware, thepossibility of doing this is unpredictable from anything heretoforeknown about the compounds, and the temperature at which the twocompounds will evaporate in the same proportions in which they arepresent in the liquid mixture can only be determined empirically for theparticular liquid mixture to be employed.

Having described my invention, I claim:

1. The method of treating melts of aluminum and its alloys comprisingmaintaining a mixture of liquid titanium tetrachloride and liquid borontrichloride in a pressure chamber in about equal proportions, heatingsaid liquid mixtureto a temperature in the range of about 45 to 60 C.,and maintaining said temperature wln'le withdrawing a stream of vaporfrom said chamber and introducing it into the metal melt.

'2. A composition consisting essentially of a liquid mixture of titaniumtetrachloride and boron trichloride in about equal proportionsmaintained in a confined space under the vapor pressure of said mixture.

3. A composition consisting essentially of a liquid mixture of titaniumtetrachloride and boron trichloride in about equal proportionsmaintained in a confined space under the vapor pressure of said mixtureand being present in amounts sufficient to maintain a substantial amountof both components of said. mixture in liquid form.

References Cited in the file of this patent UNITED STATES PATENTS

1. THE METHOD OF TREATING MELTS OF ALUMINUM AND ITS ALLOYS COMPRISINGMAINTAINING A MIXTURE OF LIQUID TITANIUM TETRACHLORIDE AND LIQUID BORONTRICHLORIDE IN A PRESSURE CHAMBER IN ABOUT EQUAL PROPORTIONS, HEATINGSAID LIQUID MIXTURE TO A TEMPERATURE IN THE RANGE OF ABOUT 45* TO 60*C., AND MAINTAINING SAID TEMPERATURE WHILE WITHDRAWING A STREAM OF VAPORFROM SAID CHAMBER AND INTRODUCING IT INTO THE METAL MELT.